Field
This disclosure relates to energy storage.
Description of the Prior Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
With decreasing battery prices, increasing battery capacity and the rise of renewable energy sources which introduce unpredictability to the energy production, battery storage for the electricity grid becomes an increasingly interesting alternative for grid balancing. The term “grid balancing” is used here to relate to the ongoing need to endeavour to match the consumption requirements of an electricity grid to the production capacity or supply capabilities associated with that electricity grid, and “grid” is used as a generic term for the electricity supply network.
Typically, the usage pattern for an electricity grid varies dramatically over the course of a 24 hour period. Overnight, usage is generally lower than during the normal working day. Within the day time period, there can be significant peaks and troughs in the consumption pattern relating to the timing of industrial activities and workers' break periods, and to the typical timing of non-working activities such as cooking an evening meal.
One effect of this variation in the consumption pattern is on the price associated with electricity supply at any particular time. The price varies in a generally inverse relation to the expected demand, so that electricity is expensive at times of high demand (or, rather, at anticipated times of high demand) and is cheaper at anticipated times of lower demand.
This pricing variation has various effects on the operation of the grid. Firstly, it provides an incentive for bulk users to draw power at times of lower demand. So, if a factory can operate overnight, its electricity costs may be considerably lower than applying to an equivalent operation during the day. Secondly, it provides an incentive for power generating companies to provide additional generating capacity at the times of higher demand.
The pricing structure is generally set in advance by an interaction between electricity providers and bulk consumers such as resellers. For example, there may be a daily interaction such as an auction which sets the prices to be imposed for electricity supply at different times during the next calendar day or 24-hour period, or the next week. These prices can then be published to the market so that consumers and providers can take whatever action they think fit based on the time-dependent pricing structure as published.
A further complication in grid balancing arrangements applies to renewable energy sources. Typically, there are strong government incentives in many countries to increase or maintain the capacity to generate electricity from so-called renewable sources such as solar, wind or wave power. While these renewable sources have many advantages in terms of their possibly lower use of non-renewable carbon resources, they suffer from a disadvantage in that the amount of electricity delivered at any instant in time can depend upon aspects of the natural environment which are outside the control of the electricity supply company. For example, the sun may go behind a cloud or the wind may drop, all leading to a relatively sudden decrease in the amount of electricity produced from a solar or wind power source. So, for times of the day when reliance is made upon renewable energy, the electricity generators need to build into the pricing model the fact that an alternative supply source may be needed at very short notice.
To address both of these needs, it has been proposed that electricity may be stored from the grid and later released to the grid at times of particular need. Various technologies are available for storing electricity in this way. In one example, one or more rechargeable batteries may be used. In other examples, mechanical energy storage may be employed, for example by connecting a large flywheel to a motor/generator arrangement, or by pumping water from a lower point to a higher point so that when the water is released from the higher point it can drive a turbine to generate electricity.